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SOX-E 18W at 50 % power The SOX-E 18W can be dimmed down to 9 W (50 % of nominal wattage) while maintaining a spectrally clean output (i.e., no neon emission). This means that the electrical discharge remains dominated by sodium vapor despite the reduced input power and burner temperature. The measured light output drops to 609.5 lm, corresponding to a lumen efficacy of 67.7 lm/W (32.3 % less than at nominal power at which level the efficacy is 100 lm/W). It is possible to reduce the input power further but neon will then show up in the lamp's optical spectrum. Note that this later variant of the lamp (2014) doesn't have any sodium-retaining dimples (it's not a Chinese knock off, I checked it). Keywords: Lamps

SOX-E 18W at 50 % power


The SOX-E 18W can be dimmed down to 9 W (50 % of nominal wattage) while maintaining a spectrally clean output (i.e., no neon emission). This means that the electrical discharge remains dominated by sodium vapor despite the reduced input power and burner temperature. The measured light output drops to 609.5 lm, corresponding to a lumen efficacy of 67.7 lm/W (32.3 % less than at nominal power at which level the efficacy is 100 lm/W). It is possible to reduce the input power further but neon will then show up in the lamp's optical spectrum. Note that this later variant of the lamp (2014) doesn't have any sodium-retaining dimples (it's not a Chinese knock off, I checked it).
DSCF0265m.jpg DSCF0281m.jpg DSCF0289m.jpg Westinghouse_H33-1-GL21Y_1m.jpg Westinghouse_H33-1-GL21Y_2m.jpg
Lamp/Fixture Information
Manufacturer:Philips
Model Reference:SOX-E 18W
Lamp
Lamp Type:Sodium low pressure
Filament/Radiator Type:Nonthermal discharge in neon, argon, and sodium vapor
Base:BY22d
Shape/Finish:Tubular clear with ITO coating
Electrical
Wattage:9.0 W
Voltage:75.8 V
Current:0.136 A
Optical
Lumen Output:609.5 lm
Lumen Efficacy:67.7 lm/W
Colour Temperature:1770 K
Colour Rendering Index:-45 Ra8
Physical/Production
Factory Location:Hamilton, Scotland
Fabrication Date:April 2014
File information
Filename:DSCF0289m.jpg
Album name:Max / Misc lamps and lighting
Keywords:Lamps
Filesize:1115 KiB
Date added:06 Apr 2026
Dimensions:1500 x 1000 pixels
Displayed:19 times
DateTime Original:2026:04:06 21:45:27
Exposure Time:1/15 sec
FNumber:f/11
File Source:Digital Still Camera
Flash:No Flash
Focal length:27.7 mm
ISO:400
Model:X-E4
Software:Adobe Photoshop 25.7 (Windows)
White Balance:0
URL:https://trad-lighting.net/gallery/displayimage.php?pid=1189
Favourites:Add to Favourites

Comment 1 to 7 of 7
Page: 1

Tuopeek   [Mon 06 Apr 2026 at 21:30]
That’s good to know these can be dimmed like that. Are you using an electronic ballast here? I wouldn’t expect the neon discharge to appear if the lamp is at the proper operating temperature but would be concerned that the temperature may fall with time allowing the neon discharge to appear at some point. If the lamps insulation is good enough to maintain the running temperature at reduces wattage that’s a neat experiment. I like how you have that lamp orientated. A lot of SOX fans will be wincing. Laughing
Max   [Mon 06 Apr 2026 at 21:47]
Yeah, I know about the position... I figured this would be OK since this is a relatively short test (less than a couple hours of runtime). However, I checked if there is any impact of the operating position on the electrical specs, and the answer is negative, so no change in the discharge properties, only on the (slow) sodium migration.

About the power supply, for all experiments shown here (the three posts) I used a standard 0.6 A SOX leak transformer fed with a variac. I adjusted the input voltage in order to get a desired current level. On the subject of the neon emission, I've always let the lamp stabilize before I took pictures, which usually required a 10-15 minute waiting period plus the extra time needed for the fine adjustments and their associated stabilizations. So, in the present case (9 W) neon did not show up in the lamp's spectrum at all, meaning that it's a valid operating power level if you don't mind the reduced efficacy.
Sammi   [Mon 06 Apr 2026 at 22:10]
Max knows what he's doing. Cool
Max   [Mon 06 Apr 2026 at 22:12]
I'm the Chuck Norris of lamps and lighting Laughing
Sammi   [Mon 06 Apr 2026 at 22:41]
You do know that he's deaded... Wonder Sad bulb icon
Max   [Tue 07 Apr 2026 at 11:00]
You're not seriously thinking that this is what I was pointing at in my humorous comment, are you?!? Ever heard of the Chuck Norris Facts?
Sammi   [Tue 07 Apr 2026 at 11:22]
I have, but I was being a bit humorous myself, 'cos I saw that he was in the list of recently died on Wiki.

Comment 1 to 7 of 7
Page: 1